[MUSIC] JOAN CARTAN-HANSEN, HOST: Gravity is the force of action between two objects.

It's called a natural force because it acts at a distance rather than needing to be in contact.

To understand gravity, you first have to define mass and matter.

Matter takes up space and has weight.

And mass is the stuff that matter is made of.

Gravity pulls on mass, but the same amount of mass can weigh differently depending on what world it is sitting on.

Astronauts could bounce easily around on the moon because the moon's gravity is less than the Earth's.

That's because the moon's mass is physically smaller.

Gravity is the force that holds us done on Earth.

It's the force that keeps the planets revolving around the sun.

Gravity is also affected by how close objects are to one another.

The Earth's gravitational pull on Pluto, isn't very strong because Pluto is so far away.

But the Earth's gravitational pull on the moon is great.

The Earth's gravity keeps the moon in Earth's orbit.

For more information about force and motion, check out the science trek website.

You'll find it at science trek dot org